Pretty crazy list but if I read the rules correctly you are allowed to use in-game cheats (anyone else read it that way?)
The following is a list of how to get a 100% completion:
* Successfully complete all 90 story and procedural missions to get 68% complete.
* Successfully complete all 30 car thefts to get an additional 2%.
* Successfully complete all 50 stunt jumps to get an additional 2.5%.
* Collect all 200 pigeons (flying rats) to get an additional 2.5%.
* Find all random characters and complete their missions to get an additional 5%.
* Successfully complete all 20 vigilante crimes to get an additional 2.5%.
* Kill all 30 people on the list to get an additional 2.5%.
* Beat friends or computers at every activity to get an additional 5%.
* Get all your friend's special abilities and do all their activities to get an additional 10%. Note: This does not include Dwayne or Roman.
So, I guess play out the regular story as fast as possible then use the cheats/maps to find all the shit?
Still some marathon work to do but could mean a couple hundred bucks on ebay lol.